(a) Field of the Invention
The present invention relates to a process for preparing rubber modified styrene resins. More specifically, it relates to a process for preparing rubber modified styrene resins which can provide molded articles having excellent impact resistance and high surface gloss.
(b) Description of the Related Art
Rubber modified styrene resins has been manufactured by an emulsion polymerization method, a continuous mass polymerization method or a solution polymerization method. The rubber modified styrene resins prepared by the emulsion polymerization is excellent in a balance between impact resistance and surface gloss, but since this kind of modified styrene resins contains large amounts of impurities such as an emulsifying agent and a dispersant, its hue is bad and its manufacturing cost is high. In addition, at the time of the manufacture, a large amount of waste water is discharged, which has an adverse influence on the environment. On the other hand, with regard to rubber modified styrene resins obtained by the continuous mass polymerization method or the solution polymerization method, there are advantages such as less impurities, a low manufacturing cost and no waste water, but the balance between an impact resistance and surface gloss is insufficient.
As techniques for improving the balance between the physical properties of the rubber modified styrene resins obtained by the continuous mass polymerization method or the solution polymerization method, for example, the following methods are known.
Japanese Patent Publication No. 7343/1974 (which corresponds to U.S. Pat. No. 3,658,946) discloses a method in which prepolymerization is carried out in a first polymerization reactor in a condition where a rubbery polymer does not invert to a dispersed phase, and polymerization is then done in a second polymerization reactor to invert the rubbery polymer to the dispersed phase (a prepolymerization method). However, in the rubber modified styrene resins obtained by this method, the diameter of rubber particles which can be controlled even by a conventional technique is large, and when acrylonitrile is added to the polymerization system, the diameter of the rubber particles further increases, so that the physical properties of the resin deteriorate. The reasons why the physical properties deteriorate are that the smaller the diameter of the rubber particles is, the higher the surface gloss of the styrene resin is, and that the impact resistance appears specifically in a suitable particle diameter range peculiar to a certain resin.
As a method capable of solving such a problem, Japanese Patent Laid-open No. 118315/1988 has suggested that conversion in each of a prepolymerization reactor and a phase inversion reactor is controlled so that ratios of the solid contents in a polymerization solution in these reactors may be close to each other. However, this method comprises complex steps, and so operativity as a practical plant is poor. Moreover, when copolymerization with the acrylonitrile is carried out, the diameter of the rubber particles cannot be sufficiently controlled, and large rubber particles are formed, so that the physical properties deteriorate.
Japanese Patent Application Laid-open No. 7708/1991 (which corresponds to U.S. Pat. No. 5,244,977) discloses a method in which a high viscous rubbery polymer having a viscosity of 400 to 2000 cP in a 5 wt % styrene solution at 25.degree. C. is used under the control of solid contents in a polymeric solution in a prepolymerization reactor and a phase inversion reactor. In this method, the rubbery polymer is scarcely dissolved in monomers at the time of the preparation of a polymeric material, and an operative problem such as the difficult stirring of the polymeric solution is present due to the high viscosity of the polymerization system. Furthermore, since extremely large rubber particles are formed, the improvement of the physical properties cannot be expected.
In EP Unexamined Publication No. 477764, another method has been suggested in which a polymerization temperature until phase inversion is maintained at 90.degree. C. or less by the use of a plug-flow type polymerization reactor, and polymerization is then carried out in the presence of polybutadiene by the use of an organic peroxide having a high graft activity, i.e., a half-life period of 20 minutes or less at 90.degree. C. to prepare a resin having a rubber particle diameter of 0.5 .mu.m or less and a high graft ratio. However, in this method, the plug-flow type polymerization reactor is used in a phase inversion step, and thus it is impossible to strictly control the rubber particle diameter, so that giant rubber particles are partially formed and surface gloss is not sufficiently improved.
In the conventional techniques described above, the multi-reactor polymerization method using two or more reactors is employed, and in the first reactor, the polymerization is carried out under conditions where the rubbery polymer does not invert to the dispersed phase. Next, in the second reactor, the rubbery polymer is inverted to the dispersed phase. However, in the second and later reactors, a process of adding the polymeric material to the polymerization system has not been employed so far.